mutt mbox format for new mailboxes

Hello; I'm a new subscriber here, but very long-time mutt user.

I believe that mutt uses mboxcl2 format for writing new mailboxes. I'd be grateful to know if that is corrrect. It would be helpful also to know how long that has been the case since I've got some 20 year old mutt mboxes I'm keen to process with a golang program.

Many thanks,
Rory